Data from powered and coast passby tests are analyzed to produce sound level versus speed information for the power train and tires as separate sources. Tires with blank, continuous rib, and discrete block tread patterns were used. Test speeds covered the range of 48 - 160 km/h (30 - 100 mph). Fit of the experimental data to a 30 log (Speed) relationship for the power train and a 40 log (Speed) relationship for the tires is exceptionally good. Some evidence of aerodynamic contribution to the rolling (coastby) sound is noted at the upper end of the speed range. These findings support the postulation that tread vibration is the primary mechanism of automobile tire sound generation.